2D Peak Position Research Articles

Graphene Fermi Level-Guided Attachment of Single Exoelectrogens and Induced Interfacial Doping.

Graphene's exceptional electronic and mechanical properties make it a promising material for bioelectronic applications; however, understanding its interaction with electrogenic bacteria is crucial to harness its full potential. This study investigates the interface between electrogenic bacteria and graphene with Raman spectroscopy by analyzing the distinctive spectral fingerprints to understand electron energy and distribution via this non-destructive and label-free method. We find that the presence of bacteria induces a distinct red-shift in the G peak positions of graphene, indicating electron doping. Correspondingly, the bacteria demonstrate a predilection for attachment on hole-rich sites on the graphene sheet, evidenced by the comparative analysis of pre- and post-spatial Raman mapping, revealing their consistent presence within the hole-doped 2D peak position range of 2673.89-2675.43 cm-1. This affinity of bacteria is due to the overall higher Fermi level (∼4.9 ± 0.2 eV) of these regions, which favors electron transfer. These findings demonstrate the potential of leveraging the graphene's electronic properties in engineering graphene-based biosensors. Tuning graphene's charge carrier concentration would enable the promotion or prevention of bacterial attachment, facilitating capture of specific bacteria or development of antimicrobial surfaces. This approach enables clean, efficient, and accurate study of graphene-based bacterial systems, driving significant advancements and enhancing their performance.

Comparison of physical and chemical vapor deposition for magnesium intercalation underneath epitaxial graphene

Intercalation of Mg into epitaxial graphene was investigated using chemical vapor deposition (CVD) and physical vapor deposition (PVD) for the formation of n-type doped graphene. Scanning electron microscopy and atomic force microscopy indicate PVD has significantly greater metal deposition and surface roughness compared to the CVD process. Findings from x-ray photoelectron spectroscopy infer the formation of Mg intercalated quasi-freestanding epitaxial graphene indicated by shifting of the SiC peak in C 1s spectra. This is substantiated by visual evidence from scanning tunneling electron microscopy of one or more layers of Mg residing underneath EG. Furthermore, an invariant 2D peak position in Raman spectra taken over a 7-day period in ambient conditions support arguments of the stability of electron doped Mg-intercalated epitaxial graphene. CVD offers a cleaner and smoother surface compared to PVD due to improved control over when the substrate is subjected to Mg exposure, but PVD allows for greater Mg concentrations per minute that may increase the amount of intercalated metal.

Gas sensing performance and charge-transfer mechanism of semiconducting single-walled carbon nanotubes

Large specific surface areas and unique monolayer atomic structures enable single-walled carbon nanotubes (SWCNTs) to exhibit highly local-environment-sensitive electrical properties, which have been demonstrated to be ideal candidates for the next-generation sensing nanodevices. However, the interfacial interaction between gas molecules and SWCNTs has not been investigated experimentally so far. Herein, (6, 5)-enriched semiconducting SWCNTs have been used to fabricate room-temperature gas sensors, which exhibits a high sensitivity of 232 % toward 1 ppm nitrogen dioxide (NO2) at room temperature. Importantly, the sensing mechanism has been first investigated according to the Raman shift of 2D peak in SWCNTs, which provides direct evidence for the interfacial charge transfer between NO2 molecules and SWCNTs. Moreover, an exponential relationship between the 2D peak position and NO2 concentration hasbeen established, which can be used to simply evaluate the charge transfer and the adsorption state of gas molecules. We believe that our results will not only promote the development of high-performance SWCNT-based sensing devices, but also provide a novel methodology to characterize the interaction between gas/vapor molecules and carbon nanomaterials.

Raman Spectroscopy Investigation on Semi-curve Woven Fabric-graphene Synthesized by the Chemical Vapor Deposition Process

Abstract: Graphene is a single layer of two-dimensional carbon atoms bound in a hexagonal lattice structure with zero band gap semiconductor. Chemical vapor deposition (CVD) is one of the most promising, inexpensive and readily ways for synthesizing monolayer pristine graphene. We have synthesized monolayer graphene shaped in semi-curve woven frabic-graphene (SWF-G) on SiO2/Si substrate. Using Raman spectroscopy, we studied the central suspended portion (i.e., 1-6) of it exerting compression (stress) to the graphene supported on the substrate. The concentration of hole impurities on either side of the central position of semi-curve woven fabric-graphene (SWF-G) is more than on its central position. The variation of such hole doping concentration results in an upshift of 2D peak position (pos(2D)) which is opposite for high electron doping even if there is no intentional control of doping. The synthesized graphene is a single-layer high-quality new structure graphene. Keywords: Semi-curve woven fabric-graphene, Raman spectroscopy, Charge impurities, Compression, Doping.

Optimization of MoCl5 intercalation for low-resistance and low-damage exfoliated highly-oriented pyrolytic graphite

This study developed a damageless molybdenum pentachloride (MoCl5) intercalation doping process of exfoliated highly-oriented pyrolytic graphite (e-HOPG). We optimized the chemical concentration and reaction temperature and time for doping crystalline multilayer graphene. We found that thick e-HOPG films are more susceptible to intercalation damage than previously reported few-layer graphene. Lowering the chemical concentration reduced the damage; however, there was a trade-off between doping efficiency and damage. Efficient doping with a 77% reduction of sheet resistance without significant damage was achieved by further optimizing the reaction temperature and time with reduced chemical concentration. The correlation plots of G and 2D peak positions in the Raman spectra were used to analyze the strain and carrier density induced during the intercalation process and investigate the cause of damage. The stability of the intercalated e-HOPG was confirmed for storage in a nitrogen box for 40 weeks.

Multimode two-dimensional vibronic spectroscopy. II. Simulating and extracting vibronic coupling parameters from polarization-selective spectra.

Experimental demonstrations of polarization-selection two-dimensional Vibrational-Electronic (2D VE) and 2D Electronic-Vibrational (2D EV) spectroscopies aim to map the magnitudes and spatial orientations of coupled electronic and vibrational coordinates in complex systems. The realization of that goal depends on our ability to connect spectroscopic observables with molecular structural parameters. In this paper, we use a model Hamiltonian consisting of two anharmonically coupled vibrational modes in electronic ground and excited states with linear and bilinear vibronic coupling terms to simulate polarization-selective 2D EV and 2D VE spectra. We discuss the relationships between the linear vibronic coupling and two-dimensional Huang-Rhys parameters and between the bilinear vibronic coupling term and Duschinsky mixing. We develop a description of the vibronic transition dipoles and explore how the Hamiltonian parameters and non-Condon effects impact their amplitudes and orientations. Using simulated polarization-selective 2D EV and 2D VE spectra, we show how 2D peak positions, amplitudes, and anisotropy can be used to measure parameters of the vibronic Hamiltonian and non-Condon effects. This paper, along with the first in the series, provides the reader with a detailed description of reading, simulating, and analyzing multimode, polarization-selective 2D EV and 2D VE spectra with an emphasis on extracting vibronic coupling parameters from complex spectra.

Abnormal Raman Characteristics of Graphene Originating from Contact Interface Inhomogeneity.

The Raman peak position shift rate per strain (RSS) coefficient of graphene is crucial for quantitative strain measurement by Raman spectroscopy. Despite its essential role, the experimentally measured RSS values are found to be highly scattered and many times significantly lower than the theoretical prediction. Here, using in situ Raman spectroscopy with a tensile test system, we resolve this controversy by examining the Raman characteristics of graphene derived from chemical vapor deposition (CVD) transferred on polymer substrates. Our experiments show that the Raman 2D-peak position of CVD graphene can shift nonlinearly with applied strain, in contrast to its intrinsically linear trait. More importantly, the resultant RSS coefficient at the steady state is much lower than the theoretical prediction. By analyzing atomic force microscopy (AFM) phase images and full width at half-maximum (FWHM) of Raman spectra, we attribute the abnormal behavior to nanometer-scale inhomogeneity of the graphene/substrate contact interface. Assisted by a simplified discrete interface slip model, we correlate the evolution of nanometer-scale inhomogeneity with that of the apparent Raman response. The theoretical model provides a useful tool for understanding and optimizing the contact interface behavior of various two-dimensional materials on substrates; the revealed mechanism is critical for correct interpretation of data obtained by Raman or any other spectroscopies based on homogenized laser signals.

Plasmon-exciton coupling for nanophotonic sensing on chip.

The monolayer graphene-noble metallic nanostructure hybrid system with excellent optical characteristic, which is deserved pay attentions in the study of surface-enhanced Raman scattering spectroscopy. In this work, a hybrid sandwich structure is designed to transfer single-layer graphene to the surface of discs substrate covered by silver film and assembly of the dense Au nanoparticles (AuNPs). Blu-ray disc has a cycle density of approximately 5.7 times that of DVD-R due to the different storage capacities of these optical discs. In the research, enhancement effects have been explored for two different periodic grating structures. Compared to spectra of Si/G structure, Graphene Raman spectra from Blu-grating/AuNPs/G structure and Blu-grating/G/AuNPs enhancement multiples at the 2D peak position possesses different Raman responses of 1.09 and 2.51 times, respectively. The sandwich hybrid structure of Ag grating/graphene/AuNPs obtains a Raman enhancement factor (EF) of 6.2×108 for Rhodamine 6G and surface-enhanced Raman Scattering(SERS) detection limit of 0.1 nM. These findings can be attributed to the electric field enhancement of the hybrid structure and the chemical enhancement of graphene. This study provides a new approach for SERS detection and offers a new technique for designing SERS sensors with grapheme-plasmon hybrid structures.

Layer dependent electrical transport in exfoliated graphene FETs under UV illumination

Controlled tunability of charge carriers is an intriguing technique to modify the electrical behavior of graphene. In this article, we report the photo-induced oxidation and its reversibility in single, bi and tri-layer graphene (S-B-TLG) devices by employing deep ultraviolet (DUV) light treatment in O2 and N2 atmosphere, respectively. Oxygen molecules in the presence of DUV light show dissociative adsorption onto the surface resulting a p-type carrier modulation in S-B-TLG devices. Treated graphene samples are characterized by Raman spectroscopy where blue-shift in G and 2D-peak positions is observed by DUV/O2 treatment without inclusion of any defect. The reverse effect of carrier modulation is observed when DUV/O2 treated samples are further studied in DUV/N2 atmosphere. The results of electrical measurements show the shift of Dirac point (DP) towards higher gate voltages (Vg > 0) which confirms a p-type carrier modulation in graphene whereas restoration of DP towards pristine state indicates the de-modulation of carriers in S-B-TLG devices. In addition, UV irradiation induced charge transport mechanism on graphene surface is further interpreted by employing density functional theory calculation and Bader charge transfer analysis. Such experimental and computational insights enhance the probability to understand the defect-free and controlled carrier modulation process, intensifying its applicability in electronic and optoelectronic devices.

Phonon anharmonicities in supported graphene

The paper presents temperature-dependent Raman studies of anharmonic phonon properties of graphene as-grown on copper, transferred to copper, SiO2/Si, and Al2O3, as well as nitrogen-doped graphene on SiO2/Si. Different G and 2D peak position and linewidth temperature dependencies were obtained in the temperature range of 20–294 K, upon which anharmonic constants for 3- and 4-phonon processes were determined. Values of anharmonic constants obtained from G peak shift for undoped graphene on dielectric substrates were quantitatively close to both experimental results for unsupported graphene and theoretical predictions reported in the literature, while the values for graphene as-grown on copper were almost two orders of magnitude greater. The results were analyzed in terms of substrate effect on phonon properties of graphene. The present study is useful for taking into account anharmonic phonon effects in graphene when designing graphene-based nanoelectronic devices.

Potassium-doped n-type bilayer graphene

Potassium-doped n-type bilayer graphene was obtained. Chemical vapor deposited bilayer and single layer graphene on copper (Cu) foils were used. After etching of Cu foils, graphene was dipped in potassium hydroxide aqueous solutions to dope potassium. Graphene on silicon oxide was characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy. Both XPS and EDX spectra indicated potassium incorporation into the bilayer graphene via intercalation between the graphene sheets. The downward shift of the 2D peak position of bilayer graphene after the potassium hydroxide (KOH) treatment was confirmed in Raman spectra, indicating that the KOH-treated bilayer graphene was doped with electrons. Electrical properties were measured using Hall bar structures. The Dirac points of bilayer graphene were shifted from positive to negative by the KOH treatment, indicating that the KOH-treated bilayer graphene was n-type conduction. For single layer graphene after the KOH treatment, although electron doping was confirmed from Raman spectra, the peak of potassium in the X-ray photoelectron spectroscopy (XPS) spectrum was not detected. The Dirac points of single layer graphene with and without the KOH treatment showed positive.

Possibility of Determining the Graphene Doping Level Using Raman Spectra

Raman spectroscopy was used to study the structure of graphene synthesized from methane by chemical vapor deposition at atmospheric pressure and transferred to a SiO2/Si substrate using various transfer and polymer-removal methods. It was found that the dependences of the 2D peak positions on the G peak positions of the studied samples were well-behaved linear functions with slopes of ~2.2 that suggested the existence of biaxial stress in the graphene. It was discovered that the doping in the samples changed.

Comparative study of single and multi domain CVD graphene using large‐area Raman mapping and electrical transport characterization

We systematically investigate the impact of granularity in CVD graphene films by performing Raman mapping and electrical characterization of single (SD) and multi domain (MD) graphene. In order to elucidate the quality of the graphene film, we study its regional variations using large‐area Raman mapping and compare the G and 2D peak positions of as‐transferred chemical vapor deposited (CVD) graphene on SiO2 substrate. We find a similar upshift in wavenumber in both SD and MD graphene in comparison to freshly exfoliated graphene. In our case, doping could play the dominant role behind the observation of such upshifts rather than the influence due to strain. Interestingly, the impact of the polymer‐assisted wet transfer process is the same in both the CVD graphene types. The electrical characterization shows that SD graphene exhibits a substantially higher (a factor 5) field‐effect mobility when compared to MD graphene. We attribute the low sheet resistance and mobility enhancement to a decrease in charge carrier scattering thanks to a reduction of the number of grain boundaries and defects in SD graphene.

Modification of the structural and electrical properties of graphene layers by Pt adsorbates

The properties of graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of platinum (Pt) metal on exfoliated single, bi- and trilayer graphene and on chemical vapor deposition-grown single-layer graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that Pt affects the structure as well as the electronic properties of graphene. The shift of peak frequencies, intensities and widths of the Raman bands were analyzed after the deposition of Pt with different thicknesses (1, 3, 5 nm) on the graphene. The shifts in the G and 2D peak positions of the Raman spectra indicate the n-type doping effect by the Pt metal. The doping effect was also confirmed by gate-voltage dependent resistivity measurements. The doping effect by the Pt metal is stable under ambient conditions, and the doping intensity increases with the increasing Pt deposition without inducing a severe degradation of the charge carrier mobility.

Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride

Raman spectroscopy, a fast and nondestructive imaging method, can be used to monitor the doping level in graphene devices. We fabricated chemical vapor deposition (CVD) grown graphene on atomically flat hexagonal boron nitride (hBN) flakes and SiO2 substrates. We compared their Raman response as a function of charge carrier density using an ion gel as a top gate. The G peak position, the 2D peak position, the 2D peak width and the ratio of the 2D peak area to the G peak area show a dependence on carrier density that differs for hBN compared to SiO2. Histograms of two-dimensional mapping are used to compare the fluctuations in the Raman peak properties between the two substrates. The hBN substrate has been found to produce fewer fluctuations at the same charge density owing to its atomically flat surface and reduced charged impurities.

Theory of the Raman spectrum of rotated double-layer graphene

We study theoretically the Raman spectrum of the rotated double-layer graphene, consisting of two graphene layers rotated with respect to each other by an arbitrary angle \theta. We find a relatively simple dependence of the Raman G peak intensity on the angle \theta. On the other hand, the Raman 2D peak position, intensity, and width show a much more complicated dependence on the angle \theta. We account for all of these effects, including dependence on the incoming photon energy, in good agreement with the experimental data. We find that it is sufficient to include the interaction between the graphene layers on the electronic degrees of freedom (resulting in the occurrence of van Hove singularities in the density of states). We assume that the phonon degrees of freedom are unaffected by the interaction between the layers. Furthermore, we decompose the Raman 2D peak into two components having very different linewidths; these widths are almost independent of the angle \theta. The change in the intensity and the peak position of one of these two components gives insight into the dependence of the overall Raman 2D peak features as a function of the angle \theta. Additionally, we show regions in the phonon spectrum giving rise to the Raman 2D peak signal. This work provides an insight into the interplay between the mechanical degree of freedom (angle \theta) and the electronic degrees of freedom (singularities in the density of states) in rotated double-layer graphene. Additionally, this work provides a way to establish experimentally the value of the rotation angle \theta using Raman spectroscopy measurement. This procedure becomes even more robust if one repeats the Raman spectroscopy measurement with a different incoming photon energy.

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.

Layered Structures of Interfacial Water and Their Effects on Raman Spectra in Graphene-on-Sapphire Systems

We investigated the structure of the interfacial water layers between graphene sheets and a sapphire substrate by observing them through graphene sheets at room temperature using atomic force microscopy. When graphene sheets were deposited at low relative humidity, the interfacial water layers appeared as small islands. They grew in layer-by-layer stacking with an increase in the relative humidity. We also investigated effects of the interfacial water layers on Raman spectra from the graphene sheets that cover the water layers. The correlation between G-peak position (intensity) and 2D-peak position (intensity) shows that the interfacial water induces hole-doping in graphene sheets. The doping density increases with increasing the amount of interfacial water. This study shows that the electrical properties of graphene sheets are tunable by controlling the hydrophilicity of substrate surfaces.

Graphene formation on step-free 4H-SiC(0001)

Step-free SiC was thermally decomposed in vacuum to better understand graphene formation in the absence of step fronts. Atomic force microscopy revealed graphene nucleating at surface pits that preferentially form along SiC{11¯00} planes. The density of these pits is 1×108cm-2, which is three orders of magnitude greater than the measured density of SiC threading dislocations. Additionally, Raman spectroscopy demonstrated that graphene on step-free regions have a redshifted 2D peak position and a smaller peak width than does graphene grown on stepped regions. This difference is attributed to film thickness, which is confirmed by cross-sectional transmission electron microscopy. Stepped regions have a graphitic film nearly 2 nm thick as compared to less than 0.7 nm for step-free regions.

Stable hole doping of graphene for low electrical resistance and high optical transparency

We report on the p doping of graphene with the polymer TFSA ((CF3SO2)2NH). Modification of graphene with TFSA decreases the graphene sheet resistanceby 70%. Through such modification, we report sheet resistance values as low as129 Ω, thus attaining values comparable to those of indium–tin oxide (ITO), while displayingsuperior environmental stability and preserving electrical properties over extended timescales. Electrical transport measurements reveal that, after doping, the carrier density ofholes increases, consistent with the acceptor nature of TFSA, and the mobility decreasesdue to enhanced short-range scattering. The Drude formula predicts that competitionbetween these two effects yields an overall increase in conductivity. We confirm changes inthe carrier density and Fermi level of graphene through changes in the Raman G and 2Dpeak positions. Doped graphene samples display high transmittance in the visible andnear-infrared spectrum, preserving graphene’s optical properties without any significantreduction in transparency, and are therefore superior to ITO films in the nearinfrared. The presented results allow integration of doped graphene sheets intooptoelectronics, solar cells, and thermoelectric solar cells as well as engineeringof the electrical characteristics of various devices by tuning the Fermi level ofgraphene.